David Cameron's promise to hold a referendum should he win the next election is not "believable", the Conservative MP behind an unprecedented move to condemn his own government's Queen Speech has said.

John Baron told the Commons on Wednesday that he would vote to leave the EU should a referendum be held today.

The Basildon and Billericay MP has tabled an amendment to the Queen's Speech that expresses regret that the government does not plan to legislate for a referendum in this parliament. The prime minister has said that is not possible because the Lib Dems would not allow it.

Instead Cameron has promised to hold an in/out referendum after the next election if he remains in Downing Street. But Baron told MPs the commitment had to be both "credible and believable" - and it was not.

"It is credible because the referendum in 2017 has an out option, but it is not yet believable," he said. "The British electorate are deeply sceptical of any politician making promises about matters European, particularly EU referendums. Too many promises have been broken in the past."

Baron said that the promise was not believable "not because we don't [Tory MPs] trust the prime minister, but because the electorate doesn't trust politicians".

To mocking laughter from Labour MPs, Baron also insisted that despite the past few days of internal-Tory Party debate over Europe, the Conservatives were "more united on this issue than we have been for a generation".

"What we disagree about is the best way convincing the electorate to the seriousness of our intent," he said.

The prime minister had hoped to undermine Baron's amendment by offering up a draft EU referendum bill to be introduced by a backbench Tory MP. However Baron said this was the "second best option" as the bill would inevitably be defeated "on a soggy Friday afternoon when nobody even notices".